Human nature has a dark and violent side. And that means you too, ladies. A new set of studies suggests it’s a Mean Girls world out there, and aggressive competition may be rooted deep in evolution.

We humans are fascinated by women willing to duke it out. Witness Jerry Springer’s career. We love to hate women who go ballistic and throw chairs at female rivals. But we also tend to marginalize aggressive behavior in women, dismissing it as aberrant. As with many of the unsavory topics I write a...